1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and, in particular, to an image forming apparatus, such as a copying machine, a printer, or a facsimile apparatus.
2. Related Background Art
With recent improvements in image forming apparatuses, there is a demand for more sophisticated techniques, and, as opposed to the conventional four-color image forming apparatuses, an electrophotographic image forming apparatus using developers of an increased number of colors has been proposed and partially realized. In addition to the conventionally used four colors of cyan, magenta, yellow, and black, other colors such as red, blue, and green, and special colors such as gold, silver, and a fluorescent color, have come to be used. In the field of the ink jet system, addition of light cyan, light magenta, etc. in general terms is becoming common practice. The objective of these innovations is to achieve a differentiation in image quality.
Image forming apparatuses using developers of an increased number of colors are of various types. Regarding an image forming apparatus using developers (toners), for example, of six colors, the following possible examples for a general use may be mentioned: a tandem type image forming apparatus 101 shown in FIG. 11, in which image formation is effected by using image bearing members (photosensitive members) in a number corresponding to the number of toner colors; an image forming apparatus 102 shown in FIG. 12, in which image formation is effected by using a single photosensitive member; and an image forming apparatus 103 shown in FIG. 13, which is disclosed in Japanese Patent Application Laid-Open No. 4-204871.
The tandem type image forming apparatus 101 shown in FIG. 11 has six image bearing members 1a, 1b, 1c, 1d, 1e, and 1f and six developing devices 41, 42, 43, 44, 45, and 46 containing developers of different spectral characteristics and arranged in one-to-one correspondence with the image bearing members, with image forming means Sa, Sb, Sc, Sd, Se, and Sf, each including a combination of one image bearing member and one developing device, being arranged in series. If used on a six-color basis, this type of image forming apparatus allows image output at the same speed, which means it is a productivity-oriented apparatus.
In the image forming apparatus 102 shown in FIG. 12, six developing devices 41, 42, 43, 44, 45, and 46 are provided for a single image bearing member 1. By rotating a rotary 4 on which these developing devices 41 through 46 are mounted, an arbitrary developer is selectively positioned at a developing portion 40 opposed to the image bearing member 1, thereby effecting image formation successively. Thus, primary transfer to an intermediate transfer member 5 is effected for each color to thereby realize multiple transfer. When transfer has been completed for all the six colors, secondary transfer to a recording material is conducted. This arrangement allows output of a six-color image with minimum requisite space.
The image forming apparatus 103 shown in FIG. 13 is a compromise between the image forming apparatuses 101 and 102 shown in FIGS. 11 and 12. Arranged in the image forming apparatus 103 are two image forming means: a first image forming means Sa including a first photosensitive drum 1a, and a second image forming means Sb including a second photosensitive drum 1b. The first image forming means Sa is equipped with developing devices 41 through 43, and the second image forming means Sb is equipped with developing devices 44 through 46. This image forming apparatus, which offers a well-balanced combination of the characteristics of the above two apparatuses, has been developed with a view toward meeting demands in terms of size, cost, and speed.
However, these image forming apparatuses 101, 102, and 103, constructed as described above, have the following problems.
First, the tandem type image forming apparatus 101 shown in FIG. 11 has a problem in that it must have a large size (lateral width). Further, each of the image forming means Sa through Sf for different developers must be equipped with a primary charger, an exposure device, a developing device, a transfer charger, etc., resulting in a rather high component cost. Thus, from the viewpoint of providing a popularly priced image forming apparatus, the above-mentioned problem is serious, which means this type is rather difficult to realize at low cost.
Next, the image forming apparatus 102 shown in FIG. 12, which uses a single image bearing member 1, has a problem in terms of output speed. To form one image, the intermediate transfer member 5 has to make six rounds, with the rotary 4 being rotated in synchronism therewith for switching between the developing devices 41 through 46. Thus, even if the rotating speed of the image bearing member 1 is the same as that in the above-described image forming apparatus 101, for example, it will be seen through simple calculation that the productivity will be reduced to ⅙. This problem is also rather serious if compact, quick, and neat image formation is to be conducted with a popularly priced image forming apparatus.
Next, the image forming apparatus 103 shown in FIG. 13, which uses two photosensitive members 1a and 1b, with the developing devices 41, 42, and 43 being secured in position around the photosensitive drum 1a and the developing devices 44, 45, and 46 being secured in position around the photosensitive drum 1b, has a problem in that there is a difference between the developing devices 41, 42, and 43 or between the developing devices 44, 45, and 46 in terms of the distance to be covered between image exposure on the image bearing member 1a or 1b and development at the developing portion 40a or 40b. When the distance to be covered between exposure and development thus differs from developing device to developing device, the electrostatic latent image undergoes a change (decay) in the while, so that it is difficult to provide developer images (toner images) of the same characteristic.
It might be possible to avoid this problem through appropriate changes in high-voltage setting, etc. However, that would involve a complicated control operation wastefully requiring a memory space for storing a program for the same and control time. This waste of time would lead to an increase in the so-called first print time, that is, the period of time between the input of an image formation start signal and the discharge of the first recording material to the exterior of the image forming apparatus.
It is an object of the present invention to provide an image forming apparatus capable of forming a high-quality image using deep-color and light-color toners of the same hue.
Another object of the present invention is to provide an image forming apparatus capable of preventing a deterioration in gradation when forming an image using deep-color and light-color toners of the same hue.
Still another object of the present invention is to provide an image forming apparatus capable of preventing a deterioration in gray balance when forming an image using deep-color and light-color toners of the same hue.
Further object of the present invention will become apparent from the following detailed description.